We have theoretically investigated the tunability of the omnidirectional reflection band ͑ORB͒ of a one-dimensional ͑1D͒ photonic crystal ͑PC͒ consisting of alternating isotropic dielectric and nematic liquid crystal layers by an external electric field. The width of the ORB becomes wide as the external voltage increases, but the center frequency is changed little. The frequency of the defect mode created by inserting a layer of liquid crystals into a 1D dielectric PC is also found to be tunable by the application of external voltage. These properties can be applied to tunable optical filters or optical switches.

We study the transmission of water waves propagating in finite-size two-dimensional periodic structures which consist of bottom-mounted cylinders using the multiple-scattering method. Complete band gaps exist between the first and the second bands in square and triangular periodic structures, as well as one modeled on a graphite atomic lattice. We investigate the dependence of the band edges on the filling fraction. The graphite-type structure shows the band gap at a lower filling fraction than the others. Therefore, the graphite-type structure may be more suitable for practical coastal protection. We also calculate the first-order force on cylinders located along the symmetry direction.

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